Disk drives are commonly used in workstations, personal computers, laptops and other computer systems to store large amounts of data that are readily available to a user. In general, a disk drive comprises a magnetic disk that is rotated by a spindle motor. The surface of the disk is divided into a series of data tracks that extend circumferentially around the disk. Each data track can store data in the form of magnetic transitions on the disk surface.
A head includes an interactive element, such as a magnetic transducer, that is used to sense the magnetic transitions to read data, or to conduct an electric current that causes a magnetic transition on the disk surface, to write data. The magnetic transducer includes a read/write gap that positions the active elements of the transducer at a position suitable for interaction with the magnetic transitions on the surface of the disk, as the disk rotates.
It is expected that users of disk drives will place ever greater demands on disk drive manufacturers with regard to the amount of data that can be stored in and rapidly retrieved from disk drive products. Modern software programs include graphics and other data structures that dramatically increase the amount of data that need to be stored. In addition, the rapid growth in the use of servers on computer networks requires large storage capabilities to accommodate the data needs of a large numbers of users on the network who utilize the servers.
Accordingly, recent disk drive research and development efforts have focused on the need to continually improve, among other things, the magnetic media used in the disks so as to substantially increase the storage capacity of each new disk drive design to levels sufficient to accommodate the ever greater demands for storage capacity placed on disk drive products by users. Moreover, the trend in recent years has been to design and build disk drive products that are lightweight and compact in size, and that operate at high rotational velocities of the disks to increase data read and write rates.
A consequence of the increasing capacity of disk drive products having compact designs is that the data density on the surface of the disks and the rotational speeds of the disks are approaching levels that are too high relative to the capability of conventional magnetic transducers to rapidly and accurately sense closely spaced, fast moving magnetic transitions in a data read operation. Moreover, conventional electronic circuits typically used to receive and process electrical signals representative of the transitions sensed by the magnetic transducers are also unable to operate at data read rates commensurate with the high data densities and rotational speeds of modern disk drive designs.
One proposal to meet the data retrieval requirements of modern disk drive designs is to utilize a magnetoresistive transducer (MR transducer) coupled to an electronic read channel that implements signal processing techniques such as partial response, maximum likelihood detection (PRML read channel). These components provide significantly improved performance capabilities and are able to process signals representative of data at rates suitable for operation with modern high capacity, high performance disk drives.
In an MR transducer, a magnetoresistive element is used to sense the magnetic transitions representing data. The magnetoresistive element comprises a material that exhibits a change in electrical resistance as a function of a change in magnetic flux of a magnetic field applied to the element. In a disk drive environment, the MR element is positioned within the transducer gap, above a disk surface. In this position, the electrical resistance of the element changes in time as magnetic transitions recorded on the disk pass beneath the gap, due to rotation of the disk. The changes in the resistance of the MR element caused by magnetic transitions on a disk occur far more quickly than the response of conventional transducers to magnetic transitions. Thus, an MR transducer is able to sense magnetic transitions at higher rotational speeds and data densities.
The MR transducer is coupled to an electronic circuit, e.g. a pre-amplifier, that operates to detect the resistance changes of the MR element, and generate electrical signals that vary in time as a function of the resistance changes. The pre-amplifier output, therefore, comprises an electrical signal that corresponds to the data recorded as magnetic transitions on the disk surface. The output of the pre-amplifier is coupled to a read channel that thereafter processes the pre-amplifier output signal according to PRML techniques to interpret the data represented by the output signal. PRML techniques can operate with more efficient data recording codes, and are able to process signals at more rapid rates than conventional peak detectors now widely used in disk drives to detect data from signals received from a transducer.
In any signal processing electronic circuit, such as a PRML read channel, the electrical signal output by the circuit varies as a function of the electrical signal input to the circuit. For example, in a simple amplifier, the signal output by the amplifier should have the same wave shape and frequency as the signal input to the amplifier, but with a higher amplitude. In other words, the amplifier amplifies the magnitude of the input signal.
The change between the input and output signals is referred to as the transfer function of the circuit. In the case of an ideal, simple amplifier, the transfer function can be expressed as output=k*(input), where k is a coefficient representing the magnitude of amplification. If the amplifier amplifies an input signal by a factor of 10, then k, in that instance, equals 10, and a one volt input signal will result in a 10 volt output signal.
Actual electronic circuits do not, however, operate in the ideal fashion that they were designed. An amplifier will introduce some "noise" into the output signal such that the output does not necessarily have the exact same wave shape and/or frequency phase as the input signal. Generally, "noise" is any unwanted electrical changes introduced into a circuit's output that may result form, e.g., certain operating characteristics inherent in the circuit design, the operating environment of the circuit, and so on.
Electronic circuits that implement or support PRML signal processing techniques are complex and include various operating parameters that affect circuit output, and thus the accuracy of data interpretation. For example, a PRML channel can include a finite impulse response filter (FIR filter) for filtering and conditioning raw signals received from an MR transducer. The FIR filter acts to shape signals received from the disk to be within desired channel response characteristics of the PRML channel for optimal performance. The FIR filter generates shaped samples from selected summations of raw samples of the signals derived from magnetic transitions on the disk. The raw samples are modified by coefficients prior to transmission to a summation node.
It is possible to test a PRML channel to determine the accuracy of data interpretation, and to vary the values of operating parameters of the component circuits comprising the channel, such as the coefficients of the FIR filter, to obtain optimized operation of the channel. In one approach, a test signal having a known pattern is written onto a disk of a PRML disk drive under test. The disk drive is then operated to use the MR transducer to read back the test signal.
The read back signal is input to the PRML channel of the drive under test. A high performance oscilloscope is coupled to the outputs of selected component circuits of the PRML channel, to receive and record the output signals. For example, the output of the FIR filter can be input to the oscilloscope. The recorded signals are then compared to the known pattern of the input test signal. By comparing the known pattern of the input signal to the various output signals received by the oscilloscope, the transfer functions of the component circuits of the PRML channel can be determined.
In the event that a determined transfer function(s) provides an output signal that does not comply with a proper or accurate interpretation of the input signal, the various parameters of the respective component circuit(s) of the PRML channel can be changed or modified in a manner to alter the respective transfer function(s). In the case of the FIR filter, the transfer function should reveal the proper shaping of the raw signals relative to the desired channel response characteristics of the PRML channel. After modification of the parameters, the drive can be tested again to once again determine the transfer function(s), and verify accurate performance by the channel. By changing and modifying parameters such as FIR coefficient values, optimized values can be found for reliable performance of the PRML channel of the disk drive under test.
As no two circuits are identical, the optimized values for parameters of component circuits of any one PRML channel will differ from the optimized values for another PRML channel. Accordingly, each disk drive should be tested during the manufacturing process to optimize the PRML channel for that drive. A problem with the above-described approach to optimizing a PRML channel is that the high performance oscilloscope required to perform testing of each drive is expensive. In a mass production disk drive assembly facility, many test oscilloscopes would be needed to test all drives in a manner that does not create a bottleneck in the assembly line. Moreover, a relatively skilled technician would be required to connect and operate each test oscilloscope.